1. Field of the Invention
The present invention is directed to a gradient optical fiber.
2. Prior Art
Since optical fibers or waveguides which have a definite profile for the index of refraction can transmit information at a high rate and with a small amount of pulse widening or broadening, more and more interest is being directed to optical fibers of this type for use in optical communication transmissions.
In an article by G. Gloge and E. A. J. Marcatili, "Multimode Theory of Graded-Core Fibers", The Bell Systems Technical Journal, Vol. 52, No. 9, November 1973, pp. 1563-1578, gradient optical fibers were discussed. The article discloses a gradient optical fiber which has an index of refraction of a formula EQU n.sup.2.sub.(r) =n.sub.0.sup.2 (1-2.DELTA.(r/a).sup..alpha.)
wherein n.sub.(r) represents the index of refraction at a given radius r with the condition 0.ltoreq.r.ltoreq.a; n.sub.0 is the maximum index of refraction at the geometric center of the fiber, .DELTA..apprxeq.n.sub.0 -n.sub.(a) /n.sub.0 and is the relative change of the index of refraction and .alpha. is an exponent characterizing the profile form. In this formula when .alpha. approaches .infin., a rectangular or stepped profile for common core cladded fiber is obtained.
The different modes that are able to expand or propagate in the fiber will have different rates of propagation. This leads to undesirable transit time differences.
In order to decrease this effect, the exponent .alpha. was therefore decreased so that the transit time differences between the modes are decreased. All modes are approximately equally rapid or fast when .alpha. is approximately 2, which occurs in a parabolic gradient fiber. The theoretic minimum for the transit time differences is obtained in accordance with the previously mentioned formula when .alpha.=2-2.DELTA..